Known plot change-over valves whose operation is switched by pilot pressurized fluids include, for example, one that comprises a main valve segment and a pilot valve segment. The main valve segment has two or more ports, valve ports into which said ports open, valve members slidably fitted in the valve ports, first and a second piston chambers respectively provided at both ends of the valve ports, and first and a second pistons to move the valve members slidably fitted in the piston chambers. The pilot valve segment supplies a pilot fluid to the first and second piston chambers to move the first end second pistons. The change-over valve thus switches the operation of the valve members through the motion of the pistons.
In this type of conventional pilot change-over valves, however, the pistons are liable to tilt when they slide in the piston chambers because the axial length of each of the pistons is shorter than the diameter thereof. The resulting eccentric load working on the pistons has caused excessive wear of sealing members and unstable valve operations. The conventional change-over valves of the type described above thus have lacked adequate durability and operational stability.
These problems can be solved by increasing the axial length of the pistons. However, increasing the axial length of the pistons necessitates increasing the axial length of the piston chambers and, as a consequence, the size of the change-over valve.